When play from hidden, does unit exhaust or ready?

According to Rule 811.1.c.3 and the related FAQs (such as FAQ #1908 and #8277), units played from Hidden follow the same rules as playing a unit from your hand.

Ruling: Units played from Hidden enter the battlefield exhausted by default.

While the act of playing a card from Hidden does open a chain (Rule 811.1.c.3), it does not change the inherent state in which a unit enters the battlefield. Unless a specific card effect explicitly states that the unit enters ready (such as the Accelerate keyword or a card-specific override), the unit will always enter the battlefield in an exhausted state.